Chemical Reversibility

Specification Point 4.13:

• Recall that chemical reactions are reversible, the use of the symbol ⇌ in equations and that the direction of some reversible reactions can be altered by changing the reaction conditions.

• Some reactions go all the way to completion and the reaction stops when all of the reactants are used up.
• In reversible reactions, the product molecules can themselves either react with each other or decompose and re-form the reactant molecules again.
• The reaction can occur in both directions: the forward reaction (which forms the products) and the reverse direction (which forms the reactants).

• When writing chemical equations for reversible reactions, two arrows are used to indicate the forward and reverse reactions.
• Each one is drawn with just half an arrowhead – the top one points to the right, and the bottom one points to the left:

A + 2B ⇌ AB₂

• The direction of reversible reactions can be altered by adjusting some or all of the following reaction conditions:
  • Temperature
  • Pressure
  • Concentration
• The direction of change depends on the chemical nature of the reactants and products and their physical states and properties.

Specification Point 4.14:

• Explain what is meant by dynamic equilibrium.

• Reversible reactions can reach a state of dynamic equilibrium.
• Equilibrium occurs when the forward and reverse reaction proceed at exactly the same rate.
• Both reactions are taking place simultaneously but there is no overall effect on the concentrations of the reacting substances i.e. their concentrations remain constant at equilibrium.
• Equilibrium is dynamic i.e: the molecules on the left and right of the equation are constantly chemically changing into each other.
• It only occurs in a closed system so that none of the participating chemical species are able to leave the reaction vessel nor can any other substances enter.

Equilibrium can only be reached in a closed vessel which prevents reactants or products from escaping system